The Payload Module

Payload carrier
Payload equipment bay
Payload management assembly
Data handling subsystem
Power subsystem

• ten instruments with very different accommodation needs for fields of view, physical layout, thermal control, etc;
• mounting surface of 43 m² externally, and 10 m² inside;
• total instruments mass of 2,200 kg;
• total instruments average power 1.9 kW over one orbit, 3 kW peak;
• attitude pointing better than 0.1° (95% probability level);
• attitude measurement better than 0.04° (95% probability level);
• handling of ten low-/medium-rate (up to 32 Mbps) and one high-rate (100 Mbps) data streams.
  

Note: Acronym definitions can be found in the Glossary.

These requirements are met by the payload module, which has two functional subassemblies in addition to the payload instruments themselves:

• the payload carrier (PLC);
• the payload equipment bay (PEB).

Payload Module

Payload carrier

The PLC provides the main structural support and harness for externally mounted instruments. In addition to this, the PLC provides accommodation for the star tracker optical assembly, the gyromechanical assemblies, the steerable Ka-band antenna, and the propulsion subsystem thrusters which are part of the PPF service module, but located on the PLM. The structure is composed of four similar sections, each 1.6 m in length (five were foreseen for the original multimission platform).

Payload equipment bay

The payload equipment bay flight model

The PEB (the flight model is seen here) has a U-shaped configuration consisting of the zenith and side panels of the lower three sections of the payload module. It provides the functions for data management, communications, and electrical distribution, and houses the equipment (belonging to either platform or instruments) mounted internally on the nadir and side panels of the three lower sections of the payload module. It provides all necessary functions to support, operate and control the payload instruments as well as all functions to handle the scientific data stream, by means of three subsystems:

• payload management assembly;
• data handling subsystem;
• power subsystem.

The working of the three subsystems in the payload equipment bay

Payload management assembly

This subsystem performs the command and control functions for the payload instruments. It comprises the following main elements:

• payload management computer (PMC) with mission specific application software;
• three remote terminal units (RTUs) which control and monitor all PEB functions;
• eight digital bus units (DBUs) which provide the link to the dedicated payload instruments;
• the RTUs and the DBUs are connected by the on-board data handling (OBDH) bus to the PMC.

Data handling subsystem

This subsystem is responsible for the management and transmission to ground of the science data streams. It includes:

• the high-speed multiplexer (HSM), which assembles the payload science data into a continuous data stream;
• on-board data recording composed of three 30-Gbit tape recorders and a 60-Gbit solid state recorder, allowing for intermediate storage of HSM output data during periods without direct ground station coverage;
• the encoding and switching unit (ESU), to encode and switch HSM, tape recorder play-back, or payload high-rate data to different downlink channels;
• X-band transmission subsystem, with three 100 Mbps links for direct transmission to ground, comprising:
      -the modulators;
      -the traveling wave tubes (TWT) and their power conditioners for signal amplification;
      -the output-multiplexer (OMUX), which combines the three links prior transmission;
      -the fixed antenna;
• Ka-band transmission subsystem, providing three 100 Mbps links for data transmission to ground via a data-relay satellite. It uses similar modulation and amplification hardware as the X-band subsystem, but requires an outboard assembly at the zenith panel consisting of a deployable mast carrying the antenna pointing mechanism (APM), shown here in stowed configuration, which controls the antenna pointing to the relay satellite according to a defined trajectory.

Ka-band antenna

Power subsystem

This comprises two power distribution units (PDU), one for the PEB and the other for the payload and the heater switching unit (HSU) to control all PEB internal temperatures.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry